The present invention involves a process that can be used to produce oxide ceramic powders for various applications. Some of the most commercially important oxide ceramics have the perovskite structure. Perovskite compounds have the general formula ABO.sub.3, where the A cation is relatively large and of low valence (such as Ba.sup.2+, Sr.sup.2+, Ca.sup.2+, Pb.sup.2+, La.sup.3+, Sm.sup.3+, Nd.sup.3+, Bi.sup.3+, K.sup.+, etc.), and the B cation is relatively small and of high valence (such as Ti.sup.4+, Zr.sup.4+, Sn.sup.4+, W.sup.6+, Nb.sup.5+, Ta.sup.5+, Fe.sup.3+, Mn.sup.3+, Mg.sup.2+, Zn.sup.2+, Ni.sup.2+, etc.). perovskite ceramics have numerous commercial applications, mainly because of their useful electronic properties. These applications include: dielectric ceramics for capacitors; piezoelectric materials for transducers and sensors; electrostrictive ceramics for micropositioners and actuator devices; and transparent electrooptic ceramics for information storage and optical signal processing. A good discussion of perovskite structure is given in the June, 1988 issue of Scientific American, PEROVSKITES, pages 74 to 81, in article by Robert M. Hazen.
The perovskite structure as typified by BaTiO.sub.3 above 135.degree. C. has a cubic structure. This structure consists of a regular array of oxygen ions at the corners, small tetravalent titanium ions in the center, and big, divalent barium ions located at the face centers. In ferroelectric perovskite compounds, the perovskite structure is distorted at low temperatures, and exhibits tetragonal, orthorhombic, or rhombohedral symmetry. At higher temperatures, the structure transforms to cubic; this transition temperature from the distorted phase to the cubic phase is called the Curie point. Ferroelectric behavior is caused by distortions in the crystal lattice caused by shifts in the position of the central cation (i.e., the Ti ion in BaTiO.sub.3); this results in a displacement of the centers of positive and negative charge of the ions within the structure and thus a net (or "spontaneous") polarization of the structure. The electrical properties are significantly affected by ferroelectricity in perovskites, giving rise to useful dielectric, piezoelectric, and electrooptic properties. The electrical properties of perovskites can be tailored to those required for a specific application by the wide range of compositional substitutions that are possible. The electrical properties of perovskite ceramics are also affected by manufacturing and processing conditions, as more fully described below.
The requirements of a powder for the numerous electric applications of perovskite ceramics depend on the specific material and its application. However, in most applications, the "ideal" powder is considered to have a fine particle size, narrow or no particle size distribution, chemical homogeneity, controlled stoichiometry, equiaxed particle shape, and to be agglomerate free. After a powder has been prepared, several processing steps are required to form the powder into a shape and to densify it into a finished functional electrical ceramic element. A powder is first formed or compacted into a partially dense shape called a green body. The exact shape depends on the electrical element's intended function and application, e.g., an electromechanical transducer or a multilayer ceramic capacitor. Once the powder is formed into a green body (e.g., by dry pressing or tape casting), the part must be densified by hot pressing, sintering, or the like. Sintering involves heating the green body to high temperature and allowing densification to occur by diffusional processes. The sintering conditions, e.g., time, temperature, pressure, and atmosphere, are dictated by the nature of the starting powder, the powder compaction, and the desired microstructure (e.g., grain size, microstructural uniformity and distribution of secondary phases) of the electrical ceramic elements., Some characteristics of the microstructure which can significantly affect the electrical properties of the ceramic element include grain size, grain size distribution, amount and location of porosity, pore size and distribution, and controlled distribution of secondary phases. Sintering is a key aspect of the manufacturing process of ceramic elements and must be controllable to insure that the production of high quality ceramic materials is reproducible. However, reproducibility of the sintering process and the ceramic element is highly dependent on the reproducibility of the powder production.
Dielectric ceramics, such as barium titanate (BaTiO.sub.3) and other titanate based compositions, are used widely for various types of capacitors (multilayer, chip, disk, grain boundary layer, etc.). These dielectric ceramics are important because they have very large dielectric constants, and the compositions and ceramic microstructures can be designed so that the dielectric constant is relatively temperature-independent. The desired electrical properties for capacitor applications can be achieved in BaTiO.sub.3 ceramics by solid solution additions of other perovskite compounds such as SrTiO.sub.3, CaTiO.sub.3, BaZrO.sub.3, and BaSnO.sub.3, or through addition of dopants such as Mg, Ni, La, Bi, Sm, Ng, Ta, etc. The properties of BaTiO.sub.3 ceramics are further improved by optimizing the microstructure (i.e., grain size, microstructural uniformity, controlled distribution of secondary phases, etc.).
Dielectric powders currently used in multilayer ceramic capacitors are prepared by conventional ceramic processing techniques, and thus require high sintering temperatures (&gt;1300.degree. C.). These powders also have large particle size thus requiring that the thickness of each dielectric layer in a multilayer ceramic capacitor must be at least 25 microns. An advanced dielectric powder with fine particle size (&lt;1 micron) and lower sintering temperatures (&lt;1100.degree. C.) would be beneficial. The finer particle size will allow for improved volumetric efficiency of the capacitor, and the lower sintering temperature will allow the use of less-expensive internal electrode materials.
Perovskite ceramics based on BaTiO.sub.3 are also useful for several sensor applications, i.e., PTCR devices. The BaTiO.sub.3 ceramic grains can be made semiconductive by doping with appropriate amounts of elements such as Nb, or La. These semiconducting BaTiO.sub.3 ceramics exhibit an increase of resistivity of several orders of magnitude of the Curie temperature. The temperature range of this resistivity anomaly in BaTiO.sub.3 can be shifted by compositional substitutions of Sr (to shift the Curie point to lower temperature) or Pb (to shift the Curie point to higher temperature). This positive temperature coefficient of resistivity (PTCR) effect can be utilized in several sensor/control or heating element applications.
The piezoelectric effect, is a tensor property that relates a microscopic strain (or displacement) of a material with an applied electric field. The piezoelectric effect is useful for several transducer and sensor applications. Very strong piezoelectric effects can be induced in ferroelectric perovskite ceramics, ceramic, by application of an electric field. The electric field polarizes (or "poles") the ceramic by partially aligning the directions of spontaneous polarization within each grain of the ceramic, resulting in a net polarization and piezoelectric activity. Most piezoelectric ceramic applications are based on perovskite solid solutions between PbZrO.sub.3 and PbTiO.sub.3, or Pb(Zr,Ti)O.sub.3. The term PZT is used herein to describe the entire family of powders comprised of lead, zirconium, titanium and oxygen as principal elements and including such compounds wherein some of the principal elements have been replaced by other elements such as dopants and solid solution substitutions.
Compositional modifications can be made to PZT to tailor the piezoelectric properties for specific applications. For instance, the precise Zr/Ti ratio impacts the location of the composition relative to the morphotropic phase boundary, and thus has a large impact on the properties of PZT. Also, the piezoelectric properties are significantly affected by dopant additions such as iron, manganese, lanthanum, antimony, niobium, and tantalum. Solid solution substitutions of barium or strontium (for lead) and tin (for zirconium) can be made to PZT to further alter the piezoelectric properties.
Commercial PZT ceramic parts manufacturers have experienced high rejection rates which can be related to poor batch-to-batch reproducibility of the PZT powder. Lower sintering temperatures of PZT powders would significantly reduce the problem of PbO volatility, and simplify the ceramic processing. Property enhancements would also be expected if PZT powders with more homogeneous solid solutions and more uniform dopant distribution were available.
Relaxor ferroelectrics are a relatively new class of PbO-based complex perovskites, with the general formula Pb(B.sub.1,B.sub.2)O.sub.3, where the B.sub.1 cation can be one of several low valent cations (e.g., Mg.sup.2+, Zn.sup.2+, Ni.sup.2+, Fe.sup.3+, etc.), and the B.sub.2 cation is of higher valence (e.g., Nb.sup.5+, Ta.sup.5+, W.sup.5+, etc.). These materials have promise for dielectric (e.g., capacitor), piezoelectric, and electrostrictive actuator (e.g., micropositioner) applications, depending on composition.
Compositions of interest for dielectric applications are based on PbMg.sub.1 /.sub.3 Nb.sub.2 /.sub.3 O.sub.3 (PMN) with solid solution additions of PbTiO.sub.3 and/or PbZn.sub.1 /.sub.3 Nb.sub.2 /.sub.3 O.sub.3 (PZN). PMN-based ceramics have higher dielectric constants than the BaTiO.sub.3 -based dielectrics, and thus have the potential for improved volumetric efficiency. In addition, these PbO-based ceramics sinter at lower temperatures (&lt;1000.degree. C.). so that when used in multilayer capacitor applications, the use of less expensive electrode materials will be possible.
Electrostriction is a phenomenon that occurs in all materials, and relates strain to an applied electric field. It differs from piezoelectricity in that the electrostrictive strain is proportional to the square of the electric field, whereas piezoelectric strain is directly proportional to the electric field. In most materials, electrostrictive strain is extremely small and thus cannot be used in transducer applications. However, the electrostrictive strains generated in some relaxor ferroelectrics are comparable with piezoelectric strains in PZT ceramics. Electrostrictive materials can be used in devices where more precise motion control is required. Compositions used for electrostrictive devices are based on PMN in solid solution with PbTiO.sub.3.
The ceramic processing of relaxor ferroelectrics by conventional milling and calcination techniques is difficult, and this has limited their applications potential. For example, it is extremely difficult to produce PbMg.sub.1/3 Nb.sub.2/3 O.sub.3 by conventional mixed oxides processing due to the formation of a stable Pb-niobate pyrochlore phase during calcination. Repeated calcination at high temperature (1000.degree. C.) is required to form the PMN powder. Another complication of conventional mixed oxides processing arises from the required high calcination temperature; the volatility of PbO alters the stoichiometry and prevents complete reaction. A two-step formation sequence in which the columbite MgNb.sub.2 O.sub.6 is first formed and then reacted with PbO to form PMN has been developed. However, the requirement to first produce a precursor powder complicates the processing and limits the ultimate process control. Advanced powder preparation techniques (such as coprecipitation) have not been successful in the preparation of phase-pure PMN ceramic powders.
Perovskite ceramics based on lead lanthanum zirconate titanate, (Pb,La)(Zr,Ti)O.sub.3 or PLZT, are useful because they can be prepared in transparent form with good electrooptic properties. The electrooptic effect relates to a change in refractive index with an applied electric field. Thus PLZT electrooptic ceramics can be used in several optical applications, including shutters, modulators, displays, color filters, image storage devices, and linear gate arrays for optical data processing.
The key to achieving transparency in PLZT ceramics is to produce a pore-free ceramic with uniform microstructure. Starting with a PLZT powder (which can be prepared by several methods), transparent PLZT ceramics are typically produced by hot pressing or liquid-phase sintering. Hot pressing involves the application of pressure at high temperature. The pressure enhances the densification, and pore-free PLZT ceramics can be prepared. With the liquid phase sintering technique, an excess of PbO is added to the PLZT powder prior to sintering. The PbO melts during sintering, forming a liquid phase which facilitates densification into a pore-free ceramic. As sintering takes place, the excess PbO evaporates from the ceramic; the sintering operation is then carried out until none of the excess PbO remains.
The first step to both of the above fabrication techniques, powder processing, is crucial to the optical quality of the final transparent PLZT ceramic. The optical quality of PLZT ceramics produced from conventionally prepared powders is limited. Improvements in optical quality of hot pressed PLZT ceramics have been demonstrated using chemically coprecipitated PLZT powder. However, both the prior art methods suffer from agglomeration and purity limitations.
Several investigators have reported the use of a hydrothermal treatment step to produce anhydrous crystalline products, including perovskite compounds. Recently, emphasis of research has been on dielectric barium titanate compounds and piezoelectric lead zirconate titanate (PZT) compounds. These investigations have all shown that sub-micron crystalline products can be formed.
It was reported by K. Abe et al, U.S. Pat. No. 4,643,984, that perovskite compounds with the general formula ABO.sub.3 could be produced using a three step procedure. The first step involved subjecting a mixture of A and B hydroxides to hydrothermal reaction in an aqueous media. Next, an insolubilizing agent, such as carbon dioxide, was added to the reacted mixture so as to precipitate unreacted A element materials to adjust the A to B stoichiometry. This step was necessary due to the soluble nature of the A elements, including lead, strontium, calcium, barium and magnesium, under the conditions of the hydrothermal treatment. The mixture formed after the second step contains both a B-rich crystalline oxide phase formed during the hydrothermal reaction and an A-rich non-crystalline, non-oxide phase formed during the second step. Alternatively the product slurry of the hydrothermal reaction was first filtered and washed, and then added to an aqueous medium containing the supplemental A elements. The product stoichiometry could then be adjusted by adding an insolubilizing agent. The final step was to filter and wash the product with the corrected A to B elemental ratio. This process was demonstrated for the preparation of compounds containing the A elements listed above and the B elements titanium, zirconium, hafnium, tin.
Although the process described by Abe et al was shown to result in the formation of compounds with the desired stoichiometries, several problems are expected from the method of production. The primary problem is the method chosen to control the A to B elemental ratio. It would be much more desirable to produce a compound in the hydrothermal treatment step which is already a full solid solution of the exact desired stoichiometry. The second step in the process described above not only adds impurities which can be detrimental to the ceramic sintering step, it also introduces inhomogeneities to the product. The washing steps are expected to remove some of unreacted A elements. This problem is most severe for compounds containing lead and strontium on the A-site.
Several investigators have reported a similar process for producing perovskite compounds in which the salts, or in some cases hydroxides or carbonates, of many of the A and B constituents are combined in an aqueous mixture. The mixture is adjusted to a basic pH through the addition of an alkaline material or ammonia. This mixture is then reacted under hydrothermal conditions to produce the crystalline perovskite compounds. The product slurry is cooled, filtered and washed with water to remove impurities remaining from the salts and the pH adjusting compounds. Examples of processes which employ these general steps have been reported by Fuji Titan Kogyo Co., Japanese patent number JP61031345 Yonezawa, et al U.S. Pat. No. 3,963,630, and D. Watson et al, Proceedings of the First International Conference on Ceramic Powder Processing Science, Orlando, 1987.
The Japanese patent reported that barium and strontium titanate could be produced by this method with high yield and complete incorporation of the A site elements, strontium and barium, presumably at a reaction pH of much greater than seven. However, complete incorporation of strontium and barium is possible as long as the reaction is operated under alkaline conditions and in the absence of chloride. The problem with this approach, however, is that other elements including lead and antimony could not be completely incorporated because of the presence of anionic impurities (chlorides or nitrates) and due to the alkaline pH condition. Another problem is the introduction of unsuitable quantities of sodium impurities into the reaction product which arises from the high concentration of sodium hydroxide employed in the reaction.
In the second investigation, by Yonezawa, et al, complex lead zirconate titanate compounds were produced. In this process, an acidic aqueous solution of the positive elements consisting of lead, titanium, zirconium, manganese, antimony, niobium, and tantalum was prepared with predetermined mole ratios. The solution was neutralized by use of NaOH, KOH or NH.sub.4 OH to a neutral or slightly basic pH. The mixture was then directly reacted in an autoclave at temperatures between 150.degree. C. and 300.degree. C. The resultant product was cooled, and the precipitate was filtered from the solution and washed to remove impurities. The product was reported to have a high yield although the filtrate was analyzed and found to contain concentrations of unreacted lead, titanium,, zirconium, manganese and antimony ions of 30, 40, 800, 400 and 1500 parts per million. For electronic applications, these solution losses are highly significant and can adversely affect electrical properties due to loss of control over product stoichiometry. The solution losses are a direct result of the anionic impurities left in the reacting solution. Other problems associated with the solution losses include disposal of hazardous effluents or increased plant complexity to provide for recovery of these elements. Because the cationic impurities were not removed before the hydrothermal reaction, it is expected that the products would contain excessive amounts of sodium or potassium. These impurities are detrimental to the sintering properties of the powder and the electrical properties of the sintered ceramics. Finally, because the ionic impurities were not removed in the above examples, exotic materials of construction would be required for the hydrothermal reactor to prevent corrosion associated with high temperature aqueous solutions containing chloride, nitrate and ammonium ions. This increases equipment costs and increases the possibility of product contamination.
In the work by Watson, et al, the formation conditions for lead titanate were determined under hydrothermal conditions. Only analyses of the lead titanate products were reported. Filtrate solutions were not analyzed with respect to lead and titanium ion concentrations, and therefore no conclusion can be made on the yield of the process. However, high levels of lead are expected to remain in the solution phase in the presence of high concentrations of either chloride or hydroxide ions in the hydrothermal reaction. This results in a product with a poorly defined A to B stoichiometry. The products formed by Watson, et al were particles of non-uniform shape and size. These type of particles are typical if chloride ions are not removed from the precipitated solution prior to hydrothermal reaction.
Another hydrothermal process for production of PZT compounds was described by K. Beal in a presentation at the American Ceramic Society Conference in Boston, August 1986. In this process, the zirconium and titanium were dissolved and neutralized. The resultant mixed hydroxide precipitate was then filtered and washed extensively to remove all traces of ionic impurities. The hydroxide gel was then mixed with lead oxide and reacted in an aqueous slurry by a hydrothermal reaction. It was determined that at a temperature of 300.degree. C., reaction to the desired perovskite crystalline powder would not occur unless significant quantities of mineralizers were added. These mineralizers included the fluorides and hydroxides of potassium, sodium and lithium. These mineralizers were shown to introduce significant concentrations of impurities to the resultant PZT products. These are expected to be detrimental to the sintering and electrical properties of the target ceramics. Also, the problem of hydrothermal corrosion is expected to be severe in the presence of such mineralizers.
Kutty, et al have described the preparation of several perovskite materials including PZT (Materials Research Bulletin, Vol. 19, pp. 1479-1488, 1984), strontium titanate (mater. Res. Bull., Vol. 22, pp. 641-650, 1987), and Ba(Ti,Zr)O.sub.3 (mat. Res. Bull., Vol. 22, pp. 99-108, 1986). In this work, hydroxide gels were prepared by neutralization of an acidic salt solution of the B elements. The gels were washed to remove ionic impurities and were mixed with oxides or hydroxides of the A elements. The slurries were then reacted under hydrothermal conditions to form the sub-micron powders of the desired compounds. The concentrations of unreacted A and B elements were not determined in this work, so it is impossible to discern whether complete reaction took place. In fact, excess amounts of A element were added, and the products were then leached with acid to remove water insoluble byproducts and to adjust the product A to B stoichiometry.
Other investigators have employed organic precursors as a feed material for a hydrothermal synthesis process. These materials add excessive cost to the process and also introduce carbon based impurities which are detrimental to the sintering properties of perovskite compounds. Examples of such processes include those reported by K. Abe, et al, U.S. Pat. No. 4,643,984.